1. Field of the Invention
The subject invention generates novel zeotype framework microporous crystal species via reverse microemulsions in a hydrothermal synthesis process. The subject product's morphology and crystallite size is controlled by the reaction volume.
2. Description of the Background Art
The continued growth of nanotechnology is inherently dependent on the development of new materials and new synthetic methodologies to control morphology on the nano-scale. As many systems such as quantum dots, zeotypes, molecular magnets, and related area rely on complex host-guest interactions during synthesis, there is a need to better understand how these interactions guide the ultimate outcome of the synthesis.
The utility of nanoporous and mesoporous materials having porosity on the order of molecular dimensions and above as adsorbants for gas and liquid separations, catalysts, ion-exchange materials, and biomimetic materials is clear and continues to proliferate rapidly. It has been estimated that refining efficiencies gained by processes based upon nanoporous catalysts save the US approximately 200 million barrels of crude oil imports per year. New nanoporous materials are finding their way into new chemical processing routes as the petrochemical industry responds to increasing foreign competition and environmental regulation. New membrane-based techniques for gas separations, reactive separations, and membrane chemical reactors, and energy storage devices require new materials having porosity on the order of molecular dimensions. Longer range potential applications of nanoporous materials include molecular electronic and electro-optic devices. Discovering new, three-dimensional nanoporous inorganic networks is of key importance to these developing technologies. This requires developing a scientific understanding of the mechanisms of formation of open-framework inorganic compounds and in the understanding of the subtle interactions of the inorganic-organic host-guest complexes involved in their formation. A second crucial factor for the development of nano-scale devices is control over the crystallite morphology of these materials such that control over the orientation of the pore systems with respect to the external environment can be controlled. Current research in zeotypes synthesis that is ongoing around the world is directed at the development and extension of suitable routes for porous materials. The subject invention presents one novel and very useful approach in increasing the rate of discovery of open-framework, nanoporous materials.
Microporous crystals with pore sizes near molecular dimensions, such as zeolites (or zeotypes in general wherein the standard silicon is replaced with other equivalent metals) and molecular sieves (microporous metal oxide crystals in general), are widely used in shape-selective catalysis and separations, and are being developed for applications in membranes, sensors, and optics. (Advanced Zeolite Science and Applications (Ed.: J. C. Jansen), Elsevier, New York, 1994 and M. E. Davis, Ind. Eng. Chem. Res. 1991, 30, 1675-1683.) Because many emerging applications of microporous materials require precise control of crystal size and orientation, (A. Kuperman, S. Nadimi, S. Oliver, G. A. Ozin, J. M. Garces, M. M. Olken, Nature 1993, 365, 239-242 and S. Feng, T. Bein, Science 1994, 265, 1839-1841.) there is significant interest in developing new strategies to control crystal structure and morphology. Reverse microemulsions have been used to control the size and shape of some inorganic materials by confining the reaction within surfactant assemblies. (M. P. Pileni, Langmuir 1997, 13, 3266-3276 and M. Li, H. Schnablegger, S. Mann, Nature 1999, 402, 393-395.) Microporous zincophosphates have also been crystallized at room temperature from reactants enclosed in reverse microemulsions. (P. K. Dutta, M. Jakupca, K. S. N. Reddy, L. Salvati, Nature 1995, 374, 44-46 and R. Singh, P. K. Dutta, Langmuir 2000, 16, 4148-4153.) Zeolites and molecular sieves usually require hydrothermal synthesis conditions (T>100° C.), at which microemulsion formation is difficult to achieve.
The framework structures of zeolites (or zeotypes in general with either silicon or suitable non-silicon equivalents) are complicated networks of interconnected Si—O rings and/or cages, with substitutions of heteroatoms (Al, B, Fe, Ga, Ti, and the like) onto Si sites (or non-silicon equivalent sites). Of the approximately 104 crystallographically and chemically reasonable three-dimensional networks of interconnected tetrahedral vertices predicted by graph theory, only 121 have been synthesized or found in nature, with a few more being synthesized each year. To prepare open-framework zeotypes an organic cation guest molecule (usually referred to as a “structure-directing agent” or SDA) is employed. The SDA directs the formation of molecular-scale inorganic-organic precursors that lead to the nucleation, growth, and crystallization of open-framework materials. The SDA is incorporated into the crystalline matrix, filling the void volumes of channels and cages, and balancing charge of the framework. The SDA is usually removed after crystallization by combustion or pyrolysis, as the SDA is very tightly confined within the surrounding structure. Most of the discoveries of open-framework topologies over the last six to eight years are the result of an increased understanding of the role of the SDA has in preorganizing the inorganic building blocks and stabilizing the open-framework structure once formed.
The subject invention techniques explore new ideas in manipulating the ways in which the synthesis is conducted to step away from the conventional approaches being followed by the zeolite synthesis community. The subject approach controls the reaction volume and medium during the initial growth stages to modulate structure and morphology to produce new framework topologies.
The foregoing information reflects the state of the art of which the applicant is aware and is tendered with the view toward discharging applicant's acknowledged duty of candor in disclosing information which may be pertinent in the examination of this application. It is respectfully submitted, however, that none of the disclosed information teaches, suggests, implies, or renders obvious, singly or when considered in combination, applicant's claimed invention.